The goals of this project are to characterize lymphocyte Fc receptors genetically and functionally. Recent findings indicate that the Fc receptors of B cells and T cells but not UL cells nor macrophages are associated with Ia antigens. In addition, a subpopulation of UL cells, the effector cells in antibody-dependent cell-mediated cytotoxicity, was shown not to bear Ia antigen. The case of the B cell, the Fc receptors and Ia antigens appear to be distinct molecules, and the association appears to be ligand induced. There is also a single polymorphic non-H-2 locus which determines at least 4 distinct antigens associated with the Fc receptor. This locus is closely linked to the M locus and more distantly linked to the Sas-1 locus, placing it on mouse chromosome 1. Studies with pharmacologic agents suggest that B lymphocyte Fc receptors are affected by the lymphocyte cytoskeleton, but that cytoskeletal structures do not play a role in the interaction between Ia antigens and Fc receptors. These findings indicate that Fc receptors may play a key role in immunoregulation.